The oil and gas industry uses a number of collection and storage containers for its sampling and related activities. For example, a certain type of container in which the sample is allowed to flash two phase is used to collect surface liquid and gas samples, usually at a separator, but also at other locations such as downstream of a choke or at a wellhead. In specific instances where a single phase bottom hole sample has been collected against nitrogen for pressure maintenance, the long term storage container it will be transferred into will also be a single phase bottle with a nitrogen charge fir pressure maintenance. In all these cases, the working space of the storage containers, i.e. where the sample resides, will have some sort of mixing device to assure that a homogeneous sample is moved out of the container when needed. The mixing devices, for example, may be spheres of different sizes which will roll back and forth when the storage container is rocked to deliver the necessary mixing action. The mixing device can also be a vortex ring which is a circular ring with an internal cone so designed that the entire ring will slide back and forth in the storage container when rocked to give the desired mixing action.
An issue that has recently surfaced associated with capture and storage containers has been hydrogen sulfide (“H2S”) adsorption. After sample capture, the stainless steel container will adsorb low levels of H2S from the sample, and a subsequent analysis of the sample will give a misleading reading of the H2S content due to adsorption loss. H2S is a hazardous chemical, and it is imperative that an accurate accounting of its concentration be undertaken. This is especially significant when designing surface facilities to handle the produced reservoir fluids.
To this end, storage containers are coated with some protective layer of material that offers an impermeable barrier to H2S adsorption. Two commonly used coatings are silicon and ceramic-based. These coatings are usually laid down in extremely fine thicknesses of 1 micron or less, and as such are susceptible to abrasion. As the mixing device moves within the storage container during mixing, however, the resulting friction causes abrasions along the protective coating due to the metal-to-metal contact of the mixing device and container wall. As a result, H2S adsorption may occur, thus resulting in erroneous WS concentration measurements.
Accordingly, there is a need in the art for less abrasive mixing devices to address the short-comings of the prior art.